In low-power dc-dc converters, current sensing or measurement is not only used for protection from overload condition. It is often utilized for increasing converter efficiency through multi-mode operation and improving dynamic response. Generally, the measurement methods can be categorized as voltage drop and observer based methods. In voltage drop based methods, a current passing through a sense-resistor or a MOSFET is extracted from the voltage drop it causes. The observer-based systems usually estimate current from the voltage across the power stage inductor.
In most cases, the existing methods are not well-suited for the integration with rapidly emerging digital controllers of switch-mode power supplies (SMPS) for battery-powered portable devices, communications computers, consumer electronics, and other applications where the overall size, the system cost, and the overall efficiency are among the main concerns. The voltage drop methods either decrease efficiency of the converter or require a wide-bandwidth amplifier, which are very challenging to realize in the latest CMOS digital processes. This is due to very limited supply voltages of standard digital circuits (often in sub 1V range), at which traditional analog architectures cannot be used. Hence, bulkier and less reliable multi-chip solutions each requiring a sensing circuit and controller implemented in different IC technologies. On the other hand, the observers suffer from a limited accuracy. Typically, the current estimation relies on prior knowledge of the inductance and equivalent series resistance values, which depend on operating conditions and change under external influences, such as aging and temperature.